Coil Binding And Bump Rubbers - Chassis

The curiosity is still there, possibly stronger than ever. The most asked question among asphalt racers is: How can I convert my more-conventional setup to running soft springs and a large sway bar-the Big Bar, Soft Spring setup? I have written two articles on the subject, against my better judgment and inclination, because I have to address technical issues at hand whether I like the trend or not. But I recently had a revelation about bump rubbers and its ugly cousin, coil bind.

2/12The low and flat attitude is both desired and common these days. We will explore its merits, how it is possible, and some of its drawbacks.
Chuck Gonzalez

A Test That Went Wrong, and Right I was involved with a redesign of a Daytona Prototype and we tested the car at a handling track, as opposed to a Daytona with high banks. We had developed what we thought was a balanced setup similar to one that had been tested before and worked very well. Why do I spend time with road racing cars? To learn things that might cross over to circle track racing. And learn I did.

First off, we ran our setup and it was tight-very tight. This was a surprise. Then our guest engineers on the test, none other than Steve Cole and his assistant with Pratt and Miller, the famed race car engineering firm out of Michigan, had us install the setup they had developed into the car. I was immediately concerned. From my analysis, the car was going to be very loose with the front end rolling much more than the rear. I even told the driver, Buddy Rice of IndyCar fame, to be careful for that reason. Surprise.

The car went out, it went faster and it was not loose. In fact, a lot of good things were happening including better grip on exit, better high speed control, and more. How could this be? I thought about it overnight and in the morning before we were to test again, the answer came to me. The relationship between what had just happened to this DP car was exactly what happens to our stock cars when we go into coil bind or onto bump rubbers.

The DP car was rolling onto a short bump rubber they call a packer at the outside front wheel in each slow turn. This increased the spring rate on that corner to what we estimated to be several hundred pounds and the roll stiffness increased reducing the front roll angle to nearly the same as the rear, making the car neutral in dynamic balance. Halleluiah! I knew I was onto something here.

Subsequent calculations done on a spreadsheet format confirmed that the increased spring rate of the outside front wheel had sufficiently altered the roll characteristics so that the setup worked. Did Steve anticipate those results? I don't know, I haven't spoken with him as of the time of this writing, but I suspect since he had developed his setups before knowing the packer gap and bump rubber rate associated with the amount of shock travel we would see, the answer might be no.

What This All Means In our world of stock car racing, we have trends that come about and hang around for five years or more. The current trend of running big sway bars and soft springs has been here longer than that, but is growing among racers who don't necessarily understand how it works. The truth is, none of us understands it completely. Now I have a perspective.

3/12The whole idea of soft springs and soft sway bar came directly from Cup racing. Unfortunately, we don't have the engineering support to accurately model our cars, or do we? You can plan out your setup to work if you understand the basics of the BBSS setups and how the arrangement of springs causes the effect to work.
Bob Bolles

I have always suspected how it all works and why anyone would want to go there in the first place. Dig out those first two articles and you'll see my thoughts on it. But now things are different, I have a means to explain how it works and exact calculations to prove it.

The goal in all of this is, by admission of numerous teams who live by this sword, is to lower the front of the car to the point of running a level attitude devoid of roll and where the front valance is restricting air from flowing under the car. This attitude is supposed to create more downforce in the front end, increase the front grip to turn the car, and serve to lower the center of gravity thereby reducing load transfer and keeping more load on the left-side tires. And it does do all of that, if done correctly.

Before you get all giddy and run out to the shop to get started converting your setup to the BBSS, I need to explain one problem associated with those setups. They are very hard to maintain any consistency with. The envelope for being dynamically balanced is narrow. That's not my personal opinion, that is the feedback I get from many racers who attempt to run these setups. Many go back to more conventional setups and get rid of the frustration.

Is Zero Roll Angle Possible? I had someone ask me the other day, "Can we really achieve zero roll angle? Doesn't every car roll to some extent even with a very large sway bar?" The answer is yes and no. It is in the arrangement of springs combined with other settings where we find the answer to his question.

The front can be made to roll very little and with a very high banking angle, near zero. We did just that way back in the late '90s at Daytona where we achieved a half degree of roll, front and rear. Then NASCAR instituted a minimum rear spring rate and we lost the method. A stock car with a 1.75 inch diameter front sway bar will roll approximately 1.3 degrees up front with 1.7 g's of lateral force and running on a 12-degree banked track. Even when the Right Front rests on a bump rubber with a spring rate of 750 pounds/inch, the roll angle only goes down to 1.0.

The trick is to utilize the banking and rear spring split to cause the rear to want to roll left the same amount. Yes, I said roll left. How do we do that? Simple, we install a much stiffer RR spring combined with a softer LR spring. This creates a compression in the LR spring that is more than the compression in the RR spring and the result is a negative roll angle, or roll to the left that is greater than the overturning moment roll angle, to create the negative roll angle once thought impossible in a stock car.

Explanation of Negative Roll Angle Negative roll angles are possible with banked tracks because the forces that influence the roll of the car in the turns (gravity and the lateral force) combine in a magnitude and direction of force that is down and to the right of the CG. This direction passes through the track surface between the rear tires creating a force pulling down on the rear of the car.

When we pull down on a suspension that has a large spring split, RR stiffer, we create a roll angle to the left. This combines with the natural roll angle to create a net roll of less than zero degrees. So, if our front end is rolling 1.2 degrees positive and the rear is rolling 1.2 degrees negative, then the result or average is zero degrees. And that is the magic of the BBSS setups.

The trick is in knowing or predicting when we have achieved equal and opposite roll angles so that our car has the desired zero roll angle. We can actually overdo the process and put too much load on the LF tire creating a serious imbalance in tire loading. The perfect setups with zero roll creates equal loading on opposing pairs of tires, LF to RR, and RF to LR.

A More Precise Example Let's use for example a typical Midwest Late Model straight rail car. We install a 1.75 inch (1 5/8 inch) diameter bar with a 1.25 hole for a wall thickness of 0.25 inch. We also install spring rates of 150 across the front with a bump rubber at the RF, a 150 LR spring and a 325 RR spring. The front roll center's dynamic location is 1.00 inch off the ground and 2.5 inches left of centerline after dive and roll, or dive only in this example. I won't tell you the Panhard bar height, but it is low.

When we model this car on a 12-degree banked track with a g-force of 1.7, the desired front roll angle, or what it would achieve if not influenced by the rear, is 0.881 degrees. With the rear spring split we see a negative roll angle of (-)0.901. Together we see where the car, if the chassis is sufficiently rigid, will attain a net roll of zero, or very close to it.

A Warning: Please remember, this is not your car. The other measurements, weight location and amounts, and more mean that you can't just install these springs and sway bar and expect it to work for you. This is just an example of how it can work. But know that it is being made to work in this very same way for some very successful teams, not only in short track, but at the top level (money wise, not necessarily interest wise) of circle track racing.

Coil Bind versus Bump Rubber In our example above, we used a bump rubber that rated at 600 pounds with the amount of loading compression, or shock travel our RF suspension would experience at a particular track. Suppose we went to a different track where the shock travel would be greater and where the RF bump rubber were compressed more so to a rate of 1,350. The front roll angle would be reduced.

When the front roll angle is reduced, in this case to 0.664, then we no longer have zero roll angle for the whole car, we have a negative roll angle and an imbalance. The higher rear roll angle, in the negative direction, will force more loading onto the LF tire and make it work too hard comparatively. If we change our rear roll angle to (-)0.660 by installing a 300 RR spring and raising the Panhard bar a bit, we will again have a zero roll angle.

When we get to this point, we see where small changes in spring rates and Panhard bar heights make relatively big differences in roll angles. It is this difficulty searching for the right roll angles front and rear that makes maintaining the BBSS setups so difficult. Yes, when they are right, they are fast on tracks that suit them. But when they are off a bit, they are bad. Consistency seems to be a problem with teams that travel to different tracks where they must search for the correct combination.

When we coil bind, we limit that corner's ability to be adjusted in spring rate. If our suspension stiffness goes to infinity, or for example's sake, 3,000 pounds/inch of rate, our sample car would have a front roll of 0.398 if the RF only is in coil bind, and 1.090 if the LF only goes into coil bind. That's a huge difference. What if both front springs go into coil bind at some point on the track and alternate back and forth, then our roll angle up front would necessarily alternate between the 0.398 and 1.090 roll angles. With both front springs in coil bind, the front roll angle would be 0.340.

The Necessity of Knowing From these examples, we now understand the importance of knowing just which corner is in coil bind or on the bump rubbers and maybe more importantly, how much. A half inch into a bump rubber might create a 115-pound rate and 1 1/2 inches into it would create a 1,550-pound rate, based on bump rubber rating I did recently.

Variable-rate springs are very unpredictable because the roll angles are constantly changing and we can't count on a known roll angle to let us design our setup. The dynamic balance as well as the handling will be all over the place. You almost have to go all the way and squash a bump rubber to maximum rate, or 1,500-plus pounds to be able to plan out a setup.

Wavering between 1/2 inch of travel and 1 inch of travel causes way too much variation in spring rate. Coil binding is one way of going directly to a known spring rate and staying there. The problem is, if the track is too rough, the coil bound corner will bounce and loose contact with the track, and with that, all traction will be lost too.

Rating Bump Rubbers I recently rated a selection of fullsized bump rubbers. These are the units that go onto the shock shaft and are compressed when the shock compresses sufficiently. They are color coded for density and here are the rates. We had, in order of stiffness, first being softest, Red, Yellow, Black, and Blue rubbers.

The Red rubber rate went from 45 at 0.25 inch to 116 at 1.00 inch, 276 at 1.50 inches, and a high of 516 at 1.75 inches. The Yellow rubber rate was 90 at 0.25 inch, 132 at 1.00 inch and 744 at 1.50 inches. The Black rubber rose to 159 at 0.25 inch, 233 at 1.00 inch, 425 at 1.25 inches and 1,548 at 1.50 inches. The stiffest rubber was the Blue one, and it was rated at 248 at 0.25 inch, 350 at 1.00 inch, 634 at 1.25 inches and 1,240 at 1.50 inches.

Some of the rubbers had a strange tendency to lose rate at a travel range of around 0.25 to 0.75 inch and I'm not sure why that was. It could have been an influence of the shock we were using. All of the rubbers maintained a fairly consistent rate from 0.25 inch to around 1.0 inch and then rose in rate significantly on up to 1.5 inches, which was near full compression.

If you cut your bump rubber down in size, the rate will increase and the distance it rises from full soft to full stiff will be reduced significantly. I tested a Black rubber that had been cut to where only two rings remained from the original four rings. The rates were 210 at 0.100 inch, 150 at 0.200 inch, 160 at 0.300 inch, 250 at 0.400 inch and 420 at 0.500 inch. At 0.5 inch, the rubber was almost fully compressed and further travel would move the rate up beyond 1,000 pounds and to infinity in only the next several 100ths of an inch.

Coil Bind Tips When utilizing coil bind, remember our example car where we went from LF coil bind to RF bind. The roll angles changed a lot from one to the other. The suspension will be solid, or very close to it, with this situation. If the track is the least bit rough or has patches or dips in the asphalt, it may not work.

One tip is to apply a rubber coating to the coils, or just one side of the coils so that when the gap between coils closes, the rubber is compressed and yields a new spring rate that helps to transition from the installed spring rate to full stiff. If you have five coils in your big spring, just a 0.100 inch coating will produce a half inch of compressible rubber.

If you run coilovers and have, say 10 coils, a 0.100 inch coating will give you a total of 1 inch of rubber spacing equal to a 1 inch bump rubber. You can rate the spring with the coating to know the spring rate per distance of travel. Knowing the amount of wheel travel and translating that to shock travel will tell you the spring rate at any amount of bind.

What Does All Of This Mean All of this information is presented to educate you just as it has us so that you can know more about what is happening when you go the BBSS route. It is not intended to coax you in that direction and I think the end result of all of this will be to either dissuade you from attempting these types of setups, or helping you to understand them enough to do it right from the start.

Does this mean I agree with this direction in setup for short track cars, NO. I probably never will encourage racers to do this to a perfectly good car. It is in many ways unnatural. It relies on the stiffness of the chassis to work correctly and if the chassis is not sufficiently stiff, then we introduce another variable into the equation for a higher level of unpredictability.

I don't like variables and I want my setups to be predictable. For those who thrive on trial and error testing, week in and week out, this is your Holy Grail. Have at it and good luck with it. As for me, call me old school, but I'll stay with something more consistent, predictable, and evidenced by the many emails, calls and indications I get across the country, consistency is winning a lot of races.

Conclusion Whether you think my way or not, ultimately does not matter. As a team, you should do what you think is best for your type of racing at your track. If zero roll and running on bump rubbers or in coil bind makes you happy, then by all means, go for it. I have always believed that most racers are not totally happy unless they are making changes and experimenting, or just plain being a scientist in our own little way in our secluded little world.

If you can't be happy at what you do, then quit. If you must experiment, and we know you must, I hope that we have provided enough useful information so that you can either find more success or at least know why you didn't. It's all in the knowing either way.